1. Field of the Invention
This invention relates generally to methods and apparatus for monitoring the blood pressure of a living subject, and specifically to the non-invasive monitoring of arterial blood pressure using acoustic techniques.
2. Description of the Related Art
Three well known techniques have been used to non-invasively monitor a subject""s arterial blood pressure waveform, namely, auscultation, oscillometry, and tonometry. Both the auscultation and oscillometry techniques use a standard inflatable arm cuff that occludes the subject""s brachial artery. The auscultatory technique determines the subject""s systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject""s mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject""s actual blood pressure waveform. Thus, true continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.
Occlusive cuff instruments of the kind described briefly above generally have been effective in sensing long-term trends in a subject""s blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.
The technique of arterial tonometry is also well known in the medical arts. According to the theory of arterial tonometry, the pressure in a superficial artery with sufficient bony support, such as the radial artery, may be accurately recorded during an applanation sweep when the transmural pressure equals zero. The term xe2x80x9capplanationxe2x80x9d refers to the process of varying the pressure applied to the artery. An applanation sweep refers to a time period during which pressure over the artery is varied from overcompression to undercompression or vice versa. At the onset of a decreasing applanation sweep, the artery is overcompressed into a xe2x80x9cdog bonexe2x80x9d shape, so that pressure pulses are not recorded. At the end of the sweep, the artery is undercompressed, so that minimum amplitude pressure pulses are recorded. Within the sweep, it is assumed that an applanation occurs during which the arterial wall tension is parallel to the tonometer surface. Here, the arterial pressure is perpendicular to the surface and is the only stress detected by the tonometer sensor. At this pressure, it is assumed that the maximum peak-to-peak amplitude (the xe2x80x9cmaximum pulsatilexe2x80x9d) pressure obtained corresponds to zero transmural pressure. This theory is illustrated graphically in FIG. 1. Note that in FIG. 1, bone or another rigid member is assumed to lie under the artery.
One prior art device for implementing the tonometry technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue, to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to at least some of the transducers. An array of different transducers is used to ensure that at least one transducer is always over the artery, regardless of array position on the subject. This type of tonometer, however, is subject to several drawbacks. First, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject""s tissue overlying the artery being sensed. This has historically led to inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and nerve damage and can restrict blood flow to distal tissue.
Prior art tonometry systems are also quite sensitive to the orientation of the pressure transducer on the subject being monitored. Specifically, such systems show a degradation in accuracy when the angular relationship between the transducer and the artery is varied from an xe2x80x9coptimalxe2x80x9d incidence angle. This is an important consideration, since no two measurements are likely to have the device placed or maintained at precisely the same angle with respect to the artery.
Perhaps the most significant drawback to arterial tonometry systems in general is their inability to continuously monitor and adjust the level of arterial wall compression to an optimum level of zero transmural pressure. Generally, optimization of arterial wall compression has been achieved only by periodic recalibration. This has required an interruption of the subject monitoring function, which sometimes can occur during critical periods. This disability severely limits acceptance of tonometers in the clinical environment.
It is also noted that the maximum pulsatile theory described above has only been demonstrated to date in excised canine arteries, and not in vivo. See, for example, Drzewiecki, G. M, et al, xe2x80x9cGeneralization of the transmural pressure-area relation for the femoral arteryxe2x80x9d, 7th Annual IEEE EMBS Conference, 1985, pp.507-510. Accordingly, the maximum peak-to-peak amplitude in vivo may not occur at the arterial pressure at which the transmural pressure equals zero. In fact, during anecdotal studies performed by the applicant herein using two prior art tonometry systems (with which several hundred applanation sweeps were recorded under numerous test conditions), the maximum pulsatile theory described above never yielded measured mean arterial pressure (MAP) that was consistently similar to the average of two cuff pressure measurements taken immediately before and after the sweep. These factors suggest that prior art maximum pulsatile theory devices may produce significant errors in measured MAP.
Yet another disability with prior art tonometry systems is the inability to achieve imprecise placement of the tonometric sensors over the artery being measured. Similarly, even if properly placed at the outset of a measurement, the movement of the subject during the measurement process may require that the sensors be repositioned periodically with respect to the artery, a capability that prior art tonometric systems do not possess. Proper sensor placement helps assure that representative data is obtained from the subject during measurement, and that accurate results are obtained.
Based on the foregoing, there is a clear need for an apparatus, and related method, for non-invasively and continually monitoring a subject""s arterial blood pressure, with reduced susceptibility to noise and subject movement, and relative insensitivity to placement of the apparatus on the subject. Such an improved apparatus and method would also obviate the need for frequent recalibration of the system while in use on the subject. Furthermore, it would be desirable to make certain components of the apparatus in contact with the subject disposable, thereby allowing for the cost effective replacement of these components at regular intervals.
The invention disclosed herein addresses the foregoing needs by providing an improved apparatus and method for non-invasively monitoring the arterial blood pressure of a subject.
In a first aspect of the invention, a method of continuously and non-invasively estimating the blood pressure existing within the blood vessel of a subject is disclosed. The method generally comprises: estimating a first pressure within the vessel; estimating a second pressure within the vessel; sensing a pressure waveform from the vessel; modeling a mechanical impulse response of the vessel as a mathematical function based at least in part on the estimated first and second pressures to derive a scaling factor; and using the scaling factor, the sensed pressure waveform, and the second pressure to estimate continuously the blood pressure within the vessel. In one exemplary embodiment, the act of estimating pressure comprises: transmitting an acoustic signal into and receiving an echo from the vessel; analyzing the echo to estimate the velocity of blood flowing in the vessel; forming a time-frequency representation of velocity; and generating an estimate of the second pressure when the time-frequency representation satisfies a given condition. In another exemplary embodiment, the act of modeling as a mathematical function comprises (i) modeling as a linear autogression function and (ii) selecting the order of the autogression function based at least in part on standard deviation and residuals.
In a second aspect of the invention, improved apparatus for continuously and non-invasively estimating the blood pressure existing within the blood vessel of a subject is disclosed. The apparatus generally comprises: a sensor adapted to detect a pressure waveform from the vessel and generate electrical signals relating thereto; a processor operatively coupled to the sensor and adapted to process the electrical signals, the processing comprising at least: (i) estimating a first pressure within the vessel; (ii) estimating a second pressure within the vessel; (iii) deriving a scaling factor by modeling a mechanical impulse response of the vessel as a function based at least in part on the estimated first and second pressures; and (iv) continuously estimating the blood pressure within the vessel based on the scaling factor, the pressure waveform, and at least one of the first and second pressures. In one exemplary embodiment, the apparatus includes both a tonometric pressure transducer and an ultrasonic transducer which, in conjunction with supporting signal processing circuitry, measure both the arterial applanation and arterial blood velocity, respectively from the radial artery of a human being. The transducers and their aassociated processing circuitry track the blood velocity in the radial artery during applanation sweeps; i.e., the time period beginning when the artery is overcompressed, and ending when the artery is undercompressed, by emitting acoustic pulses and measuring the Doppler shift in the returns or reflections of the acoustic energy from cells present in the blood. The time- frequency distribution is determined from the velocity data, as calculated by an algorithm running on a digital signal processor (DSP). The time at which the time- frequency distribution is maximized corresponds to the time at which the transmural pressure approximately equals zero, and the mean pressure read by the pressure transducer equals the MAP. The measurements of applanation and blood velocity using the apparatus are largely unaffected by the orientation of the transducers on the subject""s wrist.
These and other features of the invention will become apparent from the following description of the invention, taken in conjunction with the accompanying drawings.